Xyniae or Xyniai (Greek: Ξυνίαι) was an ancient city in Achaea Phthiotis in Greece. In the Middle Ages, it was known as Ezeros (Ἐζερός). The city was located on the western slopes of Mount Othrys, some 4 km southwest of the modern village of Xyniada (in the Phthiotis Prefecture). The city was strategically located as it controlled the passages along the nearby Lake Xynias, from Lamia to Thaumaci (modern Domokos). During the second half of the 3rd century BC the city was Aetolian, but passed to Macedonia after that, only to be plundered and its population massacred by the Aetolians in 198 BC. In 186/5 BC it passed under Thessalian control, and then under Roman rule.